As cast iron, there has been known spheroidal graphite cast iron in which graphite has a spheroidal shape. The spheroidal graphite cast iron has a tensile strength within a range of from 400 to 800 MPa, and has a tendency to decrease elongation when the tensile strength is increased and to decrease tensile strength when elongation is increased.
Recently, in a field of automobile parts or the like, where lightening is strongly required, spheroidal graphite cast iron having well-balanced mechanical properties both in tensile strength and elongation has been required. As spheroidal graphite cast iron having such mechanical properties, the following spheroidal graphite cast iron has been known.
One is a bainite spheroidal graphite cast iron obtained by heating a casting at a temperature (about 800-950° C.) for austenitizing, quenching the casting in a salt bath furnace at about 300-400° C., retaining the casting at a constant temperature in the furnace, and taking out the casting from the furnace. Another example is a bainite spheroidal cast iron obtained by adding 1-4% by mass of Ni and 0.5-1.0% by mass of Mo thereto and not subjecting to a heat treatment under a condition of, so-called, as cast.
However, in the former bainite spheroidal graphite cast iron, a sufficient bainite structure can not be attained in the case of heavy thickness products. Therefore, it is used to be adopted for a thin product. Even in this case, there is caused a problem of strain due to a thermal treatment, or a high cost due to a heat treatment using a salt bath furnace. The latter bainite spheroidal graphite cast iron has a problem of cost-increase because expensive Mo is added thereto.
If the aforementioned bainite spheroidal graphite cast iron is subjected to molten-zinc plating (for example, holding in zinc melt for 120 seconds at 460° C.) to give corrosion resistance, the bainite spheroidal graphite cast iron has a defect of decrease in tensile strength and elongation due to the heat treatment as shown in the following Table 1.
TABLE 1TensileElonga-Organi-Thermal treatment, molten-strength (MPa)tion (%)zationzinc plating treatment115012.0BainiteThermal treatment only8504.0BainiteMolten-zinc plating treat-ment after thermal treatment
Table 1 shows influence of heat (about 460° C.) on spheroidal graphite cast iron having bainite structure. Here, “thermal treatment” means holding at 900° C. for one hour and then at 380° C. for one hour, and “molten-zinc plating treatment” means holding at 460° C. for 120 seconds.
Therefore, the present invention has been made in view of the above conventional problems, and an object of the present invention is to provide a high strength and ductility spheroidal graphite cast iron having well-balanced mechanical properties both in tensile strength and elongation and having improved tensile strength and elongation than conventional one.
Another object of the present invention is to provide spheroidal graphite cast iron which is not decreased in mechanical properties even if it is subjected to a treatment such as hot dipping and which has improved tensile strength and elongation without adding Mo thereto.
The other object of the present invention is to provide a non-austemper-treated spheroidal graphite cast iron obtained without being subjected to an austemper treatment where the material is heated at a temperature for austenitizing, and then quenched at about 300-400° C., retaining the casting at a constant temperature.